List concatenation and repetition in Python - Mini Project: Build & Apply

List concatenation and repetition

📖 Scenario: You are organizing a small party and want to prepare a list of snacks to serve. You have two lists of snacks from different stores and want to combine them. Also, you want to repeat some popular snacks multiple times to make sure there is enough.

🎯 Goal: Build a Python program that creates two snack lists, combines them into one list, repeats some snacks, and then prints the final snack list.

📋 What You'll Learn

Create two lists of snacks with exact items

Create a variable for the number of repetitions

Concatenate the two snack lists

Repeat a snack list using the repetition variable

Print the final combined and repeated snack list

💡 Why This Matters

🌍 Real World

Combining and repeating lists is useful when managing inventories, menus, or any collection of items where you want to merge or duplicate entries easily.

💼 Career

Understanding list operations like concatenation and repetition is fundamental for data handling, automation scripts, and preparing data for analysis or display in many programming jobs.

1

Create two snack lists

Create a list called store1_snacks with these exact items: 'chips', 'cookies', 'nuts'. Also create a list called store2_snacks with these exact items: 'soda', 'juice'.

Python

# Create the two snack lists
# Your code here

Hint

Use square brackets [] to create lists and separate items with commas.

2

Create a repetition count

Create a variable called repeat_count and set it to the number 3. This will be used to repeat some snacks later.

Python

store1_snacks = ['chips', 'cookies', 'nuts']
store2_snacks = ['soda', 'juice']
# Create the repeat_count variable
# Your code here

Hint

Just assign the number 3 to the variable repeat_count.

3

Combine and repeat snack lists

Create a new list called all_snacks by concatenating store1_snacks and store2_snacks. Then create another list called popular_snacks by repeating store1_snacks exactly repeat_count times.

Python

store1_snacks = ['chips', 'cookies', 'nuts']
store2_snacks = ['soda', 'juice']
repeat_count = 3
# Concatenate the two snack lists into all_snacks
# Repeat store1_snacks repeat_count times into popular_snacks
# Your code here

Hint

Use the + operator to join lists and the * operator to repeat a list.

4

Print the final snack lists

Print the all_snacks list and then print the popular_snacks list, each on its own line.

Python

store1_snacks = ['chips', 'cookies', 'nuts']
store2_snacks = ['soda', 'juice']
repeat_count = 3
all_snacks = store1_snacks + store2_snacks
popular_snacks = store1_snacks * repeat_count
# Print all_snacks and popular_snacks
# Your code here

Hint

Use two print() statements, one for each list.

Practice

1. What does the expression [1, 2] + [3, 4] do in Python?

easy

A. Creates a new list combining both lists: [1, 2, 3, 4]

B. Repeats the first list twice: [1, 2, 1, 2]

C. Creates a list of two lists: [[1, 2], [3, 4]]

D. Raises a TypeError because lists cannot be added

Solution

1. Step 1: Understand the + operator with lists

   The + operator joins two lists into one new list containing all elements from both.

2. Step 2: Apply + to [1, 2] and [3, 4]

   Joining these lists results in [1, 2, 3, 4].

3. Final Answer:

   Creates a new list combining both lists: [1, 2, 3, 4] -> Option A

4. Quick Check:

   List concatenation = combined list [OK]

Hint: Use + to join lists, not to repeat [OK]

Common Mistakes:

• Thinking + repeats lists instead of joining
• Confusing list concatenation with nesting
• Expecting an error when adding lists

2. Which of the following is the correct syntax to repeat the list [5, 6] three times?

easy

A. [5, 6] ** 3

B. [5, 6] + 3

C. 3 + [5, 6]

D. [5, 6] * 3

Solution

1. Step 1: Identify the operator for repetition

   The * operator repeats a list multiple times when used with an integer.

2. Step 2: Check syntax correctness

   [5, 6] * 3 repeats the list three times: [5, 6, 5, 6, 5, 6].

3. Final Answer:

   [5, 6] * 3 -> Option D

4. Quick Check:

   List repetition uses * operator [OK]

Hint: Use * with list and number to repeat [OK]

Common Mistakes:

• Using + instead of * for repetition
• Trying to use ** which is invalid for lists
• Confusing order of operands for repetition

3. What is the output of this code?

lst1 = [1]
lst2 = [2, 3]
result = lst1 * 2 + lst2 * 3
print(result)

medium

A. [1, 1, 2, 3, 2, 3, 2, 3]

B. [1, 2, 3, 1, 2, 3]

C. [1, 1, 2, 3, 3, 3]

D. [1, 1, 2, 3, 2, 3]

Solution

1. Step 1: Calculate lst1 * 2

   Repeating [1] twice gives [1, 1].

2. Step 2: Calculate lst2 * 3

   Repeating [2, 3] three times gives [2, 3, 2, 3, 2, 3].

3. Step 3: Concatenate the two results

   Adding [1, 1] + [2, 3, 2, 3, 2, 3] results in [1, 1, 2, 3, 2, 3, 2, 3].

4. Final Answer:

   [1, 1, 2, 3, 2, 3, 2, 3] -> Option A

5. Quick Check:

   Repeat then join lists = combined repeated list [OK]

Hint: Repeat lists first, then add them [OK]

Common Mistakes:

• Adding before repeating
• Miscounting repeated elements
• Confusing order of concatenation

4. Find the error in this code snippet:

list_a = [1, 2]
list_b = [3, 4]
result = list_a * list_b
print(result)

medium

A. No error, prints [1, 2, 3, 4]

B. SyntaxError due to missing parentheses

C. TypeError because * expects an integer, not a list

D. ValueError because lists cannot be multiplied

Solution

1. Step 1: Understand list repetition rules

   List repetition uses * with an integer, not another list.

2. Step 2: Identify the error in list_a * list_b

   Multiplying two lists causes a TypeError because the right operand must be int.

3. Final Answer:

   TypeError because * expects an integer, not a list -> Option C

4. Quick Check:

   List * list is invalid, needs int [OK]

Hint: Right side of * must be integer for list repetition [OK]

Common Mistakes:

• Trying to multiply two lists directly
• Expecting concatenation with * operator
• Confusing syntax error with type error

5. You have two lists:
fruits = ['apple', 'banana']
veggies = ['carrot']
How do you create a new list that repeats the fruits twice and veggies three times, then combines them all in order?

hard

A. fruits * veggies * 5

B. fruits * 2 + veggies * 3

C. fruits + veggies * 5

D. (fruits + veggies) * 5

Solution

1. Step 1: Repeat fruits and veggies separately

   fruits * 2 repeats fruits twice; veggies * 3 repeats veggies three times.

2. Step 2: Concatenate the repeated lists

   Adding these repeated lists combines them in order as desired.

3. Final Answer:

   fruits * 2 + veggies * 3 -> Option B

4. Quick Check:

   Repeat separately then add lists [OK]

Hint: Repeat lists separately, then add [OK]

Common Mistakes:

• Repeating combined list instead of separate lists
• Using invalid multiplication of lists
• Confusing order of operations